1. Field of the Invention
The present invention relates to a semiconductor device manufactured by including the step of annealing a semiconductor film using a laser beam (hereinafter referred to as laser annealing) and a manufacturing method thereof. Note that the semiconductor device indicated here includes an electrooptical device such as a liquid crystal display device or a light emitting device and an electronic device including the electrooptical device as a part.
2. Description of the Related Art
A technique for performing laser annealing to a semiconductor film formed on an insulating substrate made of glass or the like to crystallize it or to improve crystallinity thereof is widely studied. Silicon is often used for the above semiconductor film.
Recently, in order to improve mass production efficiency, there is remarkable movement toward enlargement of a substrate such that the standard substrate size used in production lines of newly constructed factories is now becoming 600 mmxc3x97720 mm. It is difficult with a currently available technique to process a synthetic quartz glass substrate into a substrate having such a large area. Even if that is possible, it is considered that its price cannot be reduced to a level practical for industrial use. There is, for example, a glass substrate as a material capable of easily manufacturing a large area substrate. The glass substrate has an advantage such as low cost and easiness of manufacturing the large area substrate, as compared with the synthetic quartz glass substrate frequently used in the prior art. Also, a laser is preferably used for crystallization because the melting point of the glass substrate is low. The laser can apply high energy to only a semiconductor film without largely increasing a temperature of the substrate.
There is a substrate called, for example, Corning 7059 as the glass substrate. Corning 7059 is quite inexpensive, has high processability, and can be easily enlarged in size. However, the distortion point temperature of Corning 7059 is 593xc2x0 C. and a problem is caused in the case of heating at 600xc2x0 C. or higher. Also, there is Corning 1737 having a relatively high distortion point temperature as one of glass substrates. The distortion point temperature of Corning 1737 is 667xc2x0 C. and higher than that of Corning 7059. Even when an amorphous semiconductor film is formed on the Corning 1737 substrate and it is left at 600xc2x0 C. for 20 hours, a deformation of the substrate such as to affect a manufacturing process was not observed. However, the heating time of 20 hours is too long for a mass production process. Also, it is preferable that the heating temperature of 600xc2x0 C. be as lower as possible in view of a cost.
In order to solve such problems, a new crystallization method is devised. This method is described in details in Japanese Patent Application Laid-open No. Hei 7-183540. Here, this method will be briefly described. First, a trace amount of metallic element such as nickel, palladium, or lead is added to an amorphous semiconductor film. The addition method is preferably performed using plasma processing method, an evaporation method, an ion implantation method, a sputtering method, a solution coating method, or the like. After the above addition, for example, when the amorphous silicon film is left in a nitrogen atmosphere at 550xc2x0 C. for 4 hours, a crystalline semiconductor film having a preferable characteristic is obtained. Heating temperature, heating time, and the like, which are suitable for crystallization are dependent on an addition amount of metallic element and a state of the amorphous semiconductor film.
However, according to the above technique, there is a problem in that the metallic element used for promoting crystallization is left also in high resistance layers (channel forming region and offset region). Since electric current can easily flow through the metallic element, a resistance of a region which should be a high resistance layer is reduced. Therefore, an off current is increased and thus variation between respective elements is produced, which causes deterioration in the stability and reliability of a TFT characteristic.
In order to solve this problem, a technique (gettering technique) for removing an metallic element for promoting crystallization from a crystalline semiconductor film is developed and disclosed in Japanese Patent Application Laid-open No. Hei 10-270363. According to the gettering technique, first, an element belonging to group 15 is selectively added to the crystalline semiconductor film and thermal treatment is performed. By this thermal treatment, the metallic element in a region to which the element belonging to group 15 is not added (gettered region) is emitted from the gettered region to be diffused and captured in a region to which the element belonging to group 15 is added (gettering region). As a result, the metallic element can be removed or reduced in the gettered region. Further, heating temperature at the gettering can be made to be 600xc2x0 C. or lower which the glass substrate can withstand. Also, it is confirmed that even when not only an element belonging to group 15 but also an element belonging to group 13 is introduced, the metallic element can be gettered.
The crystalline semiconductor film formed through such manufacturing steps has high mobility. Thus, a thin film transistor (TFT) is formed using the crystalline semiconductor film and often utilized for, for example, an active matrix electric device.
In an active matrix liquid crystal display device, a pixel circuit for performing image display for each functional block and a driver circuit over a single substrate for controlling the pixel circuit composed of a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like formed on the basis of a CMOS circuit as the basics are formed.
In the pixel circuit of the active matrix liquid crystal display device, TFTs (pixel TFTs) are arranged for each of several tens to several millions of pixels and a pixel electrode is provided in each of the pixel TFTs. An opposing electrode is provided on an opposing substrate positioned so as to sandwich the liquid crystal therebetween to thereby form a kind of capacitor using liquid crystal as dielectric. This device is configured such that a voltage applied to the respective pixels is controlled by a switching function of a TFT to control a charge to the capacitor to thereby drive the liquid crystal, and the amount of transmitting light is controlled to display an image.
The pixel TFT is made from an n-channel TFT and used as a switching element for applying a voltage to the liquid crystal to drive it. Since the liquid crystal is driven by an alternating current, a method called a frame reverse drive is employed in many cases. Since power consumption is suppressed to be low with this method, with respect to a characteristic required for the pixel TFT, it is important to sufficiently reduce an off current value (drain current flowing at an off operation of the TFT).
A low concentration drain (LDD: lightly doped drain) structure is known as a TFT structure for reducing the off current value. In this structure, a region to which an impurity element is added at a low concentration is provided between the channel forming region and the source region or the drain region, which is formed by adding an impurity element thereto at a high concentration. This region is called a LDD region. Also, a so-called GOLD (gate-drain overlapped LDD) structure in which the LDD region is overlapped with the gate electrode through a gate insulating film is known as means for preventing deterioration of an on current value due to a hot carrier. It is known that with such a structure, a high electric field near the drain is relaxed to prevent hot carrier injection and thus a deterioration phenomenon is effectively prevented.
Also, in order to obtain the GOLD structure, end portions of the gate electrode are formed in a shape having tapers. With such a shape, a step of introducing an impurity element for imparting an n-type to a semiconductor layer composing an n-channel TFT and a step of introducing an impurity element for providing a p-type to a semiconductor layer composing a p-channel TFT are respectively performed by one doping processing. Thus, the source region and the drain region are formed in a region which is not overlapped with the gate electrode and LDD regions having concentration gradients in conformity with the shape of the tapers can be formed under the tapers of the gate electrode.
Also, energy of an ion implanted into the semiconductor film in doping processing is very large as compared with bond energy of elements composing the semiconductor film. Thus, the element composing the semiconductor film is flown from a lattice point by the ion implanted into the semiconductor film to produce a defect in crystal. Therefore, after the doping processing, in order to repair the defect and simultaneously to activate the implanted impurity element, thermal treatment is performed in many cases. As the thermal treatment, there is a thermal annealing method using a furnace-annealing furnace, a laser annealing method, or a rapid thermal annealing method (RTA method). Also, the activation of the impurity element is an important process in order to produce the regions to which the impurity element is added to be low resistance regions so that they can function as the LDD regions, the source region, and the drain region.
The element belonging to group 15 is implanted into the semiconductor film by an ion doping method (which is a method of dissociating PH3 or the like by plasma and accelerating an ion by an electric field to implant it into the semiconductor film, in which mass separation of an ion is basically not performed). When, for example, phosphorus is introduced for gettering, a necessary phosphorus concentration is 1xc3x971020/cm3 or higher. The addition of the element belonging to group 15 by the ion doping method causes an amorphous state of the semiconductor film. However, an increase in a concentration of the element belonging to group 15 hinders recrystallization by later thermal treatment and thus this becomes a problem. Also, the addition of the element belonging to group 15 at a high concentration causes an extension of processing time required for the doping, which is a problem since it results in a reduction of a throughput in a doping step.
Further, the element belonging to group 15 is an impurity element for providing an n-type. It is required that a concentration of an impurity element for providing a p-type (for example, the element belonging to group 13), which is necessary to reverse a conductivity type is 1.5 times to 3 times higher than that of the element belonging to group 15, which is added to the source region and the drain region of a p-channel TFT. Thus, there is a problem in that a resistance of the source region and the drain region is increased due to the difficulty of recrystallization.
The present invention is a technique for solving such problems, and an object of the present invention is to achieve improvement of performance characteristics and reliability of a semiconductor device represented by an active matrix liquid crystal display device manufactured using TFTs, by effectively removing a metallic element left in a crystalline semiconductor film obtained using the metallic element for promoting crystallization of a semiconductor film and by performing satisfactory restoration of crystallinity of the semiconductor film and activation of the metallic element.
The present invention is characterized in that, in order to recover crystallinity of a low concentration impurity region overlapped with a portion of the gate electrode and to activate an impurity element, a substrate on which a reflecting film is formed or a reflecting plate made of a material having high reflectance (hereinafter called a reflector) are provided in a rear side (in this specification, it is defined as a surface opposite to a surface on which a semiconductor film is formed) of a substrate on which a semiconductor film is formed (hereinafter referred to as a semiconductor film substrate) and laser light is irradiated from a front side (in this specification, it is defined as a surface on which a semiconductor film is formed) of the semiconductor film substrate and the laser light transmitted through the semiconductor film substrate is reflected by the reflector and then the laser light is irradiated again, this time from the rear side of the semiconductor film substrate. At this time, the substrate may be heated to about 450xc2x0 C. When the substrate is heated simultaneous with the irradiation of laser light, the recovery of crystallinity of the semiconductor film and the activation of the impurity element can be more effectively performed.
The low concentration impurity region described above is a region into which an impurity of one conductivity type is introduced. The element belonging to (group 15 or the element belonging to group 13 is used as the one conductivity type impurity. In addition, hydrogen may be added to the low concentration impurity region and both one conductivity type impurity and hydrogen are included in the low concentration impurity region.
Also, the element belonging to group 15 and the element belonging to group 13 may be added to the low concentration impurity region, and thus both the element belonging to group 15 and the element belonging to group 13 are included in the impurity region.
Also, the element belonging to group 15, the element belonging to group 13, and hydrogen may be added to the low concentration impurity region, and thus both the element belonging to group 15, the element belonging to group 13, and hydrogen are included in the impurity region.
Further the present invention is characterized in that a semiconductor film is crystallized using a metallic element for promoting crystallization, an impurity region to which a noble gas element (also called a noble gas) is added is formed, and the metallic element included in the semiconductor film is segregated to the impurity region by thermal treatment to thereby perform gettering, and subsequently a reflector is provided in a rear side of a semiconductor film substrate, and laser light is irradiated from a front side of the semiconductor film substrate to irradiate laser light from the rear side of the semiconductor film substrate.
When the noble gas is used, the introduction amount of impurity elements can be reduced. Thus, damages to the gate insulating film, the semiconductor film, and an interface therebetween due to doping processing can be reduced and trap centers can be decreased. Therefore, reliability in manufacture of a TFT can be improved. Also, since the trap centers are decreased, a width of an overlap region between the gate electrode and the low concentration impurity region can be shortened. Thus, a transistor can be further microfabricated.
As the noble gas element, there may be used one kind or plural kinds of elements selected from the group consisting of He, Ne, Ar, Kr, and Xe. When these ions are accelerated by an electric field to introduce it into the semiconductor film, a dangling bond and a lattice distortion are produced and thus a gettering cite can be produced.
Also, one conductivity type impurity may be added to the impurity region to which the noble gas element is added, and thus both the noble gas element and one conductivity type impurity are included in the impurity region. The element belonging to group 15 or the element belonging to group 13 is applied as the one conductivity type impurity. In addition, hydrogen may be added to the impurity region, and thus the noble gas element, one conductivity type impurity, and hydrogen are included in the impurity region.
Also, the element belonging to group 15 and the element belonging to group 13 may be added to the impurity region to which the noble gas element is added, and thus the noble gas element, the element belonging to group 15, and the element belonging to group 13 are included in the impurity region.
Also, the element belonging to group 15, the element belonging to group 13, and hydrogen may be added to the impurity region to which the noble gas element is added, and thus the noble gas element, the element belonging to group 15, the element belonging to group 13, and hydrogen are included in the impurity region.
Also, the reflector may be provided in contact with the semiconductor film substrate or may be provided to be physically separated from the semiconductor film substrate.
The present invention is also characterized in that a material which is resistant to heat and has a high reflectance with respect to the laser beam is used as a material for forming the reflector. As shown in FIG. 5, the reflector may he made of an element selected from the group consisting of aluminum (Al), tungsten (W), tantalum (Ta), titanium (Ti), chromium (Cr), and silver (Ag), a compound including the element, or an alloy including the element. A reflecting film may be formed on the substrate as the reflector. Also, a reflecting plate made of a material having high reflectance may be used as the reflector.
As regards the reflector, a surface thereof by which the laser light is reflected may be a plane surface or a curved surface. The laser light is condensed on or near the surface of the semiconductor film formed on the substrate. Also, a part of the laser light is transmitted through the substrate and reflected by the reflector to be irradiated onto the semiconductor film also from a rear side thereof. At this time, when the surface of the reflector, by which the laser light is reflected, forms a plane surface, there may be a case where the laser light reflected by the reflector becomes more spread or scattered as compared with laser light incident from the front side of the semiconductor film. Thus, when the surface of the reflector by which the laser light is reflected is formed as a curved surface, laser light which is reflected from the reflector and condensed can be irradiated from the rear side of the semiconductor film and thus an effective energy density in the semiconductor film can be further increased. Since the curvature of the curved surface is dependent on a state of laser light, a distance between the substrate and the reflector, and the like, it may be appropriately determined by an operator. Also, rugged portions may be provided on a reflecting surface of the reflector to effect diffuse reflection of the laser light.
Also, when irradiating laser light to the substrate from the front side of the substrate on which the semiconductor film is formed, the substrate and the reflector may be moved relative to the laser light. Alternatively, only the substrate may be moved relative to the laser light and the reflector.
Also, it is an essential condition that the laser tight used in the present invention be able to transmit through the substrate. FIG. 6A shows transmittance of a 1737 glass substrate with respect to a wavelength and FIG. 6B shows transmittance of a synthetic quartz glass substrate with respect to a wavelength. From FIGS. 6A and 6B, if some transmittance is required for the substrate to be used, a wavelength of the laser beam is desirably 300 nm or more (preferably, 400 nm or more). Also, from FIGS. 6A and 6B, the substrate is desirably selected in accordance with the laser to be used. For example, when an XeCl excimer laser (308 nm in wavelength) is used, since the transmittance of the synthetic quartz glass substrate is higher than that of the 1737 glass substrate, it is preferable to use the synthetic quartz glass substrate. Further, a solid laser rather than a gas laser is desirably used as the laser. This reason is as follows. That is, gas used for the gas laser is generally quite expensive, and thus when a frequency of gas exchange is high, there is a problem in that it increases manufacturing cost. Also, exchange of attached devices such as a laser tube for laser oscillation and a gas purifying unit for removing an unnecessary compound produced in an oscillation process are required once every two to three years. These attached devices are often expensive, which also causes an increase in manufacturing cost such as described above. Thus, when the solid laser (laser for outputting a laser beam using a crystal rod as a resonant cavity) such as a YAG laser is used, a running cost (here, which means a cost produced with operation) can be reduced as compared with the gas laser.
Also, when irradiating the laser light to the semiconductor film substrate from the front side of the semiconductor film substrate, the laser light may be irradiated to the semiconductor film substrate at a slant angle.
Also, when the reflector is manufactured once, it can be reused many times.
Also, there is an amorphous semiconductor film or a crystalline semiconductor film as the semiconductor film. A compound semiconductor film having an amorphous structure, such as an amorphous silicon germanium film may also be used other than the amorphous semiconductor film.
Thus, when the present invention is applied, the semiconductor film for which gettering of the metallic element, the recovery of crystallinity of the semiconductor film, and the activation of the impurity element has been satisfactorily performed can be obtained and the performance of a semiconductor device can be greatly improved. For example, in the case of a TFT, when the metallic element is sufficiently gettered, an off current value is reduced and variation in the off current value can be suppressed. Also, when the crystallinity of the semiconductor film is sufficiently recovered, the channel forming region becomes a high resistance region and a leak current can be reduced. Also, when the impurity element is sufficiently activated, the regions to which the impurity element is added are formed as low resistance regions which function as the LDD region, the source region, and the drain region.